1. Field of the Invention
The present general inventive concept relates to an inkjet image forming apparatus, and more particularly, to a line printing type inkjet image forming apparatus which prevents deviations of ink dots from original locations during time-division driving.
2. Description of the Related Art
In general, inkjet image forming apparatuses form ink images on a print medium by ejecting ink from a printhead that reciprocates in a widthwise direction that is perpendicular to a feeding direction of the print medium while being spaced apart from a top side of the print medium by a predetermined gap, thereby forming an image. Such an inkjet image forming apparatus for printing the image by ejecting ink onto the print medium while the printhead reciprocates in the direction perpendicular to the feeding direction of the print medium is referred to as a shuttle type inkjet image forming apparatus. A nozzle unit including a plurality of nozzles ejecting ink is disposed at the printhead of the shuttle type inkjet image forming apparatus.
Recently, to achieve a high-speed printing, a printhead having a fixed nozzle unit with a length corresponding to a width of the print medium has been developed to replace the printhead reciprocating in the widthwise direction of the print medium. An inkjet image forming apparatus having the printhead with the fixed nozzle unit is referred to as a line printing type inkjet image forming apparatus. The printhead of the line printing type inkjet image forming apparatus is fixed and only the print medium is moved. Thus, a unit for driving the line inkjet image forming apparatus is simple and the high-speed printing can be achieved, but when a required resolution is higher than an actual resolution of the printhead, it is difficult to print an image with the required higher resolution.
Japanese Patent Laid-open Publication No. 2001-232781 describes a conventional inkjet image forming apparatus. FIG. 1 illustrates ink dots ejected on a print medium using a conventional inkjet image forming apparatus. FIG. 2 illustrates an example of ink dots ejected on another print medium using the conventional image forming apparatus. FIG. 3 illustrates another example of ink dots ejected on another print medium using the conventional image forming apparatus. In addition, FIG. 4 is an enlarged view of a portion of print regions of the printing medium of FIGS. 2 and 3.
A printhead 20 having a plurality of nozzles N1 to NN and extending along a width of the print medium P in a direction that is perpendicular to a print medium-feeding direction (X-direction) is illustrated in FIG. 1. When the plurality of nozzles N1 to NN are sequentially driven, a deviation degree W that corresponds to a distance from a dot DD1 to a dot DDN in the print medium-feeding direction is generated on the print medium P. Here, the deviation degree W is a difference in the print medium-feeding direction between positions of the dot DD1 ejected from a first nozzle N1 and the dot DDN ejected from an N-th nozzle NN. As the deviation degree W increases, ink dots are not deposited (or ejected) at correct positions and as ink dots deviate further from the correct positions, an image quality is lowered. To enhance resolution in the medium-feeding direction of the print medium P, the print medium P should be slowly fed and printed. The deviation degree W can be reduced using the following methods: as illustrated in FIG. 2, ink may be ejected by dividing a plurality of head chips 21 into groups so that the groups are placed in a reverse order, or as illustrated in FIG. 3, ink may be ejected by disposing the plurality of head chips 21 alternately so that the head chips 21 are placed in the reverse order. However, when a time-division driving is performed in the reverse order, as illustrated in FIG. 4, the deviation degree W can be reduced, but the ink dots are not uniformly ejected. For example, two ink dots are deposited in a predetermined region 10 while ink dots are not deposited in another region 30. That is, ink dots are not uniformly deposited on the entire region of the printing medium P. Thus, a difference in an optical density occurs between the predetermined region 10 where ink dots are overlappingly deposited while the region 30 where ink dots are not deposited thereby lowering the printed image quality. This is a big problem in the conventional inkjet image forming apparatus that attempts to print high quality images. Accordingly, an inkjet image forming apparatus having an improved structure becomes necessary.